+ AP Chemistry Exam Review + Science Practices Laboratory Exercises +Gravimetric Analysis Source What It Determines: amount of analyte by mass measurements % composition empirical formulas Video Virtual La b How It Can Be Done: Dehydration of a hydrate Forming a precipitate, which is then isolated and massed Analysis: Remember to ALWAYS go to moles! use mole ratios to convert between various components Contamination in Possible Source solid of Error Impact on Results measured mass too large Science Practices 1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain. 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 4.2 The student can design a plan for collecting data to answer a particular
scientific question. 5.1 The student can analyze data to identify patterns or relationships. 6.1 The student can justify claims with evidence. Incomplete Precipitation Solid not fully dehydrated Lose ion in filtrate mass too small measured mass too large Learning Objectives 1.3: The student is able to select and apply mathematical relationships to mass data in order to justify a claim regarding the identity and/or estimated purity of a substance. 1.17: The student is able to express the law of conservation of mass quantitatively and qualitatively using symbolic representations and particulate drawings. 1.19: The student can design, and/or interpret data from, an experiment that uses gravimetric analysis to determine the concentration of an analyte Determination of Molar Volume of a Gas +Gas Laws Labs How Its Done/Analysis: Determination of Molar Mass of a Volatile Volatile liquid heated until completely vaporized Liquid PV=nRT to solve for n: Temperature of water bath=T Small hole in stopper means Pressure = room pressure Volume=volume of flask Divide Mass of recondensed unknown by moles, compare to known molar mass to ID unknown Assume vapor completely recondenses If lose vapor, then mass would behttp://chemskills.com/?q=ideal_gas_law too low, and moles would be low Diffusion : Grahams Law Demo How Its Done/Analysis:
Ends of glass tube plugged with soaked cotton White ring is NH4Cl ppt Science Practices 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 2.3 The student can estimate numerically quantities that describe natural phenomena. 6.4 The student can make claims and predictions about natural phenomena based After rxn, eudiometer lowered into water until levels equal so pressure inside room and tube are same Use Daltons Law to subtract Pwater Sources of error: Mg left: moles of H2 too low Air bubble at start: volume of gas too high Source 1 Source 2 Video 1 Video 2 Video 3 Ppt will not be exactly in middle Molar mass of NH3 is lower than MM of HCl, so ring will be toward right side of tube Grahams Law can be used to establish ratios which can be applied to distances
Learning Objectives LO 2.4: The student is able to use KMT and concepts of intermolecular forces to make predictions about the macroscopic properties of gases, including both ideal and nonideal behaviors. LO 2.6: The student can apply mathematical relationships or estimation to determine macroscopic variables for ideal Click here to +Chromatography watch animation Source explaining column chromatography Video Virtual La b Source Main Error: (includes nice discussion of biological applications) Incomplete Separation Can be caused by: Inconsistent spotting Overloading Improper packing (column) Poor solvent choice Advantage: We can collect and use the fractions Science Practices 4.2 The student can design a plan for collecting data to answer a particular scientific question. 5.1The student can analyze data to identify patterns or relationships. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. Learning Objectives LO 2.7: The student is able to explain how solutes can be separated by chromatography based on intermolecular interactions. LO 2.10: The student can design and/or interpret the results of a separation experiment (filtration, paper chromatography, column chromatography, or distillation) in terms of the relative strength of interactions among and between the components. Source +Synthesis
Inorganic Synthesis Examples: Synthesis of Coordination Compound Synthesis of Alum from aluminum Video Organic Synthesis Example: Synthesis of Aspirin Virtual La Syntheses are done in solution, and require purification by filtration, recrystalization,b column chromatography or a combination % Yield is calculated; analysis is done by melting point, NMR, IR Synthesizing Alum Procedure: Source aluminum Filter Recrystallize H Mixture isof heated added collect alum to until alum ppt inis no ice 2SO4and reacted with KOH crystals bath Al(s) is3 left Al(OH) 3 Possible Error Sources: Possible Error Sources: crystals some Possible Error Sources: impurity in Al (such
as plastic escape form Al(s) remains tootoquickly, filtrate, undissolved incomplete crystals not incomplete precipitation coating) prevents complete washed well and remain wetrxn recrystallization Science Practices 2.1 The student can justify the selection of a mathematical routine to solve problems. 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 4.2 The student can design a plan for collecting data to answer a particular scientific question. LO: Learning Objectives LO 3.5: The student is able to design a plan in order to collect data on the synthesis or decomposition of a compound to confirm the conservation of matter and the law of definite proportions. LO 3.6: The student is able to use data from synthesis or decomposition of a compound to confirm the conservation of matter and the law of definite proportions. +Titration- Acid/Base Assumption: Endpoint is equivalence point Source 1 This is not true we actually overshoot before indicator changes Possible Sources of Error: Overshoot Endpoint Not reading to bottom of meniscus Moles of titrant (and therefore analyte) too high Moles of titrant too low Concentration of titrant not what expected
Be sure to rinse buret with titrant first to control for this Science Practices 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 5.1 The student can analyze data to identify patterns or relationships. Strength of Acids affects shape of graph: Source 2 Video Source Virtual La b Polyprotic acids have multiple end points: Learning Objectives LO 6.12: The student can reason about the distinction between strong and weak acid solutions with similar values of pH, including the percent ionization of the acids, the concentrations needed to achieve the same pH, and the amount of base needed to reach the equivalence point in a titration. LO 6.13: The student can interpret titration data for monoprotic or polyprotic acids involving titration of a weak or strong acid by a strong base (or a weak or strong base by a strong acid) to Source 1 + REDOX Titration Endpoint can be Or Or REDOX an determined by indicator: indicator that potentiometer: shows the presence of a particular species: Applications/ Methods of REDOX Titrations: Vitamin C Content Video Virtual La b Source Starch indicates
presence of iodine Concentration of a redox species Source Source Back Titrations Science Practices 4.2 The student can design a plan for collecting data to answer a particular scientific question. 5.1 The student can analyze data to identify patterns or relationships. Done when the endpoint can be hard to ID add an excess, and then back titrate to determine how much is in excess Learning Objectives LO 1.20: The student can design, and/or interpret data from, an experiment that uses titration to determine the concentration of an analyte in a solution. LO 3.9: The student is able to design and/or interpret the results of an experiment involving a redox titration. Virtual Lab 1 + Kinetics Using Spectrometry: Clock Reactions Reactions that have a delayed physical change due to mechanism which only has a later step show color change Concentration vs. time data can be gathered Time (s) Data: Zero Order Plot 1st Order Plot 2nd Order Plot Virtual Lab 2 Video Source 1 It is important that there is a relatively low [S2O3-] so that the I3- will accumulate and show the color change
Analysis rate law from straight-line graph k from integrated rate law Ea from Arrhenius Equation Science Practices 2.1 The student can justify the selection of a mathematical routine to solve problems. 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 5.1 The student can analyze data to identify patterns or relationships. Blue in the presence of starch Learning Objectives LO 4.2: The student is able to analyze concentration vs. time data to determine the rate law for a zeroth-, first-, or second-order reaction. LO 4.3: The student is able to connect the half-life of a reaction to the rate constant of a first-order reaction and justify the use of this relation in terms of the reaction being a first-order reaction. + Calorimetry Application of Law of Conservation of Energy: All heat produced/consumed during reaction (system) is exchanged with the surroundings. Applications: Coffee Cup Calorimeter Assumption: the calorimeter is isolated so the surroundings are only the calorimeter setup, which includes the water the calorimeter itself has a heat capacity, as it will absorb some heat. So Constant P qsurr = -qrxn qsurr = qcal+qH O+sys qcal = CcalTT 2 Therefore: -qrxn= CcalTT + mH2O+syscTT Then we can use qrxn find THSource rxn: Science Practices 1.4 The student can/mol use representations and models to analyze TH =q rxn or rxnproblems react situations solve qualitatively and quantitatively. 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 4.2 The student can design a plan for collecting data to answer a
particular scientific question. 5.1 The student can analyze data to identify patterns or relationships. Source 1 Specific Heat of Material Can be used to ID unknown Heat of reaction Energy content of food Video Virtual La b Source This is usually done in a bomb calorimeter (constant V) Learning Objectives LO 5.4: The student is able to use conservation of energy to relate the magnitudes of the energy changes occurring in two or more interacting systems, including identification of the systems, the type (heat versus work), or the direction of energy flow. LO 5.7: The student is able to design and/or interpret the results of an experiment in which calorimetry is used to determine the change in enthalpy of a chemical process (heating/cooling, phase transition, or chemical reaction) Virtual Lab Virtual Lab 1 2 + Qualitative Analysis Remember that the lower the Ksp, the less soluble the substance. Qualitative analysis then also allows a ranking of salts by Ksp, and demonstrates the effect of pH on solubility. Science Practices 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 2.3 The student can estimate numerically quantities that describe natural phenomena. 5.1 The student can analyze data to identify patterns or relationships. 6.4 The student can make claims and predictions about natural phenomena Video Source Learning Objectives LO 6.21: The student can predict the solubility of a salt, or rank the solubility of salts, given the relevant Ksp values. LO 6.22: The student can interpret data regarding solubility of salts to determine, or rank, the relevant Ksp values. LO 6.23: The student can interpret data regarding the relative solubility of salts in terms of factors (common ions, pH) that influence the
+Write This, Not That Write This Not That! Rationale Generally The language used in the question when asked Other words that may mean the same thing to make a choice (ex: increases, decreases, but are likely more ambiguous (ex: goes etc.) up, goes down, etc.) Make it easy to give you points, and be sure the reader can understand what you saying Answer the specific question first, then justify, explain etc. Burying the answer in the text of the response Make it easy to give you points names of specific elements and compounds, reactants, products, etc. it Ambiguous Species It, stuff, etc. Be formal in language A justification or explanation when it is part of the question Only the answer without supporting it Justification/explanation required to earn point mass, volume, etc. size Be specific References to specific data or graphs when prompted to explain how the data or something similar Make generalizations about the data without Required to earn point specifically citing provided data or trials Net ionic equations only containing species that Aqueous ionic compounds in their change undissociated form, spectator ions Including these is not a net ionic, its a molecular or complete ionic Particle view diagrams with ions and polar molecules orientated in the correct direction relative to each other Drawings must demonstrate understanding of interactions at the molecular level (ref. 2015 #4)
If include units is written in the prompt, a unit is required to earn full points Work is often what earns some/all of the points 1 pt traditionally is assessed somewhere in the FR for significant figures. Incorrectly oriented dipoles An answer with units if include units is stated An answer without units in the problem Show all work used to derive an answer An answer without supporting work shown Answers expressed to the correct number of significant figures Answers with an incorrect number of significant figures Write + This Not That! Rationale Gases Components of the Kinetic Molecular Theory as Ideal gas law for molecular level justification arguments based on PV = nRT are at justifications for changes at the molecular level the bulk level and not the molecular level (ref. 2013 #5) Thermodynamics Values with correct signs Values with incorrect signs Necessary for correct calculations and determinations watch signs based on bonds breaking/forming, heat flow in calorimetry indicated by temperature changes, signs that may change in application of Hess Law, etc. Value of k without units Collision must occur in the correct orientation A rate law without k being included Units required to earn point AP wants more specific answer Kinetics Value of k with units Specific parts of the molecules that must collide in order for the reaction to occur A rate law that includes the rate constant k as part of it A rate law based only on reactants A rate law that includes products Incomplete rate law if k is not included Rate laws are based only on reactants Equilibrium Discussion of Q vs. K
Proceeds Ksp expressions that only contain the ions Correct formulas (including charges!) for all species in equilibrium expressions In Kp expressions: Pspecies x has been assumed to be so small relative to reduce the stress, or due to Le Chteliers Principle Shift if equilibrium has not yet been established (i.e. a precipitate has not yet been formed when evaluating Ksp) Preferred AP language If equilibrium is not yet established, then it cannot shift rxn will proceed in a certain direction until equilibrium is established Solids and liquids are not included in Ksp expressions that contain or imply a equilibrium expressions species in the denominator Substitutions, abbreviations, chargeless ions, Equilibrium expressions must be other shorthand that may work out in written formally when requested calculations but does not represent the correct species In Kp expressions: [species] Concentration is not used in Kp, partial pressures are Nothing about why you ignore x to avoid Show you understand why you are +Write This, Not That continued Write This Not That! Rationale The pH > 7 because its a battle between weak acid and strong base and strong base wins. The solution is neutral when pH=7. State the actual reason not the memory aid Acids and Bases The pH > 7 because the salt produced in the neutralization behaves as a base: A- + H2O HA + OH- The solution is neutral when [H3O+] = [OH-]. Kw = Ka x Kb for a conjugate pair Kw = Ka x Kb for an unrelated acid/base pair pH = pKa because it is at the equivalence pH = pKa point of a titration of a weak acid with a strong base True definition of neutral neutral is only pH of 7 when Kw = 1.0 x 1014 (at 298 K) This equation only holds true for conjugate acid-base pairs Explains the reason behind this, and shows you understand this is only true at this point Atomic Structure
Effective nuclear charge increases It has a more polarizable cloud of electrons period Reference reasons for periodic trends (i.e. effective nuclear charge, coulombs law, polarizability, etc.) Electrons in higher energy levels are farther from the nucleus, resulting in a larger atom/ion. It wants to have a full octet; its close to having a full octet It has more electrons, it has more mass, it has more surface area, it is bigger, it has more protons shell when referring to elements and their location on the Periodic Table Stating the trend as the reason (because it is to the left, because it is further down the periodic table, etc.) More electrons/more energy levels make the atom/ion bigger. State the actual reason not the memory aid This is the shortest way to show the reason simply mentioning more of something is probably not enough to demonstrate without further explanation of why that is the case Elements are in a period, electrons are in a shell State the actual reason not the memory aid Explanation of reason, not just statement of fact, required for point (Ref 2016 #1) Write This + Bonding and Intermolecular Forces Overcome intermolecular forces Ion interactions Not That! Rationale break up a solid/liquid LDFs when discussing ionic compounds IMFs should be used to justify Ionic compounds have ions with whole charges, which dominate interactions State the actual reason not the memory aid State the actual reason not the memory aid Coulombic attraction Opposites attract Describe the process of overcoming intermolecular forces/polarity Has hydrogen bonds between the
molecules Like dissolves like ionic compound molecule when discussing an ionic compound atoms when discussing ionic compounds ions when discussing covalent compounds Lewis structures that are missing lone pairs and/or resonance (if needed for correct structures) stronger intermolecular forces ions atoms Lewis structures that are complete with necessary lone pairs and/or resonance Has hydrogen bonds Identify specific intermolecular forces at play dissolve when discussing interactions ionize, dissociate, bond, react, between molecular substances in solution attack, break up, etc. Shows that you understand hydrogen bonds are not actually bonds A molecule is a covalent compound Ionic compounds contain ions Covalent compounds do not contain ions Lewis structures are incorrect without necessary lone pairs Shows your understanding of the chemistry at play Molecular substances do not dissociate into ions, dissolving is not reacting, and otherwise be formal in usage Electrochemistry Loss of mass of electrode is due to atoms Loss of mass of electrode is due to loss Electrons have extremely small (negligible in this case) mass of electrode going into solution as ions of electrons Discussion of Q vs. K for changes in cell potential after a change, or qualitative discussion of Nernst Equation Discussion of Le Chteliers principle (ref. 2014 #3) Preferred AP language (ref. 2014 #3)
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